The advent of Electrospray Ionization (ESI) has expanded the utility of Mass Spectrometry (MS) immensely, initiated the development of novel bio-analytical applications and further supported the advancement of existing analytical methods, particularly approaches associated with liquid chromatography. Fundamental aspects of the ESI process governing the formation of gas phase ions have been discussed and debated over a vast amount of experimental data and theoretical considerations, whilst ongoing investigations are focused on enhancing sensitivity, address suppression effects and introduce design refinements to the ESI source and MS interface. Yet the coupling of the ESI source to a mass analyzer has proved a rather perpetual task, and involves extending the operation of ion optical systems to intermediate pressures, requires consideration of the gas dynamics of under-expanded flows established in the fore vacuum region of mass spectrometers and furthermore necessitates the identification of key design parameters of the ESI source to enhance performance.
Progressive iterations of the original ESI source design involve methods to improve sampling efficiency using multi-capillary inlets or multiple-aperture configurations [U.S. Pat. No. 6,803,565 B2 Smith 2001; U.S. Pat. No. 7,462,822B2, Franzen 2006; U.S. 2011/0127422A1, Hansen 2009], operation at reduced flow rates using multiple emitters [U.S. Pat. No. 7,816,645B2, Kelly 2008] and elevated temperatures to promote desolvation [U.S. Pat. No. 7,199,364B2 Thakur 2007] as well as the construction of pneumatically assisted ESI emitters [U.S. Pat. No. 7,315,021B2, Whitehouse 2005] to aid droplet fission, accommodate higher flow rates and minimize structural unfolding thus control the charge state distribution of high mass ions [Takats et al, Anal Chem 76, 4050-4058 (2004); Wang et al, J Am Soc Mass Spectrom 22, 1234-1241 (2011)]. In other versions the ESI is conceptualized with post ionization capabilities using photons [U.S. Pat. No. 7,109,476B2 Syage 2004] or reagent species [U.S. Pat. No. 8,080,783B2 Whitehouse 2009] as means to address suppression effects enhance sensitivity and selectivity. Other critical parameters such as the position and distance of the sprayer probe relative to the inlet in combination with pneumatic nebulization and curtain gas flows has produced a diverse set of ESI source designs, which is supported by an extensive body of literature. Nevertheless, regardless of any novel design aspects being implemented to enhance performance the vast majority of ESI sources is still attached externally to vacuum and thus the overall ion transfer efficiency, which is practically dictated by the narrow dimensions of inlet capillaries or apertures is estimated to fall below <1% [Page J. et al, J Am Soc Mass Spectrom 18, 1582, (2007)]. Approaches to increase the size of the conductance-limiting inlet system to the mass spectrometer are expected to increase ion transmission and have a significant impact on sensitivity, however, improvements are limited by the practical constraints imposed by the requirement for greater pumping speed. Here, the upper operating pressure threshold established in the fore vacuum region of a mass spectrometer must also be considered as a limitation and currently set to 40 mbar (30 Torr), which is the highest operating pressure of the ion funnel [Kelly R T et al, Mass Spectrom Rev 29, 294-312 (2010)].
The concept of electro-spraying directly inside the fore vacuum region as a means to circumvent the severe ion losses that occur at the atmospheric-pressure interface of a mass spectrometer was proposed early on the development and proliferation of the ESI source [U.S. Pat. No. 5,115,131 Jorgenson et al 1991; U.S. Pat. No. 6,068,749 Karger et al 1997; Gamero-Castano et al, J Appl Phys 83(5), 2428-2434 (1998); Romero-Sanz & de la Mora, J Appl Phys, 95(4), 2123-2129 (2004); Marginean et al, Appl Phys Lett 95, 184103 (2009)], however, there have been very few attempts to prove the practical aspects of such an approach for bioanalytical applications [Page et al, Anal Chem 80, 1800-1805 (2008); Marginean et al, Anal Chem 82, 9344-9349 (2010)]. This first successful implementation comprises of an ESI source operated at low flow rates (<0.5 μL/min) and directly coupled to an ion funnel. The device is known as the sub-ambient pressure ionization (SPIN) source [U.S. Pat. No. 7,671,344B2 Smith et al 2007; U.S. Pat. No. 8,173,960B2 Smith et al, 2009]. The latest design of the SPIN source is a revised version of the original configuration where a first vacuum compartment operated at elevated pressure (˜40 mbar) compared to a second vacuum compartment enclosing the ion funnel is introduced. First and second vacuum compartments are in communication via a conductance-limiting aperture of 2-5 mm. A gas supply is used to admit the bath gas in the first vacuum compartment. CO2 or SF6 gases are usually employed to suppress arcing and allow application of the high voltage necessary for the ESI process to ensue.
In spite of the successful implementation of ESI at sub-ambient pressures, the current design of the SPIN source is limited to low flow rates as a result of incomplete desolvation of charged droplets produced at the emitter tip. The residence time inside the first vacuum compartment operated at elevated pressure is short and dictated by the mobility of the charged droplets in the presence of high electric fields established between the ESI tip and the conductance limiting aperture, separated by a few mm only. Ion losses are also expected in the conductance limiting aperture unless sizes greater than several mm wide are used, in which case the pressure differential can no longer be maintained unless substantial temperature gradients are established. Another important limitation of the existing technology is that the construction of the ion funnel prevents from being driven to elevated temperature to promote desolvation. Furthermore, although desolvation and liberation of gas phase ions from charged droplets is possible in the presence of RF fields, the RF field free region established over a significant volume of the ion funnel limits desolvation to near the terminating aperture of the system only.
Higher flow rates in sub-ambient ESI may be achieved with a pneumatic nebulization system employing an ionization process. However, severe ion losses can be expected due to the radial dispersion of charged droplets in the absence of a gas flow re-focusing mechanism. Strong radial velocity components enhance diffusion of charged droplets and product ions significantly affecting instrument sensitivity thus are severely problematic in this regard.